1. Field of the Invention
The present invention generally relates to monitoring the motion of a scan mirror employed for sweeping a light beam in laser scanning arrangements, such as image projectors for displaying images or electro-optical readers for reading indicia and, more particularly, to reducing contamination in a feedback signal indicative of such mirror motion.
2. Description of the Related Art
It is generally known to project a two-dimensional image on a projection surface based on a pair of scan mirrors which oscillate in mutually orthogonal directions to scan a laser beam over a raster pattern comprised of a plurality of scan lines. The image is created in the raster pattern by energizing or pulsing a laser on and off at selected times, thereby illuminating selected pixels with a beam spot and not illuminating other pixels in each scan line.
One of the scan mirrors, sometimes referred to herein as an X-mirror, sweeps the laser beam at a relatively faster speed generally along a scan direction extending along the horizontal, and the other of the scan mirrors, sometimes referred to herein as a Y-mirror, sweeps the scan line at a relatively slower speed generally perpendicular to the scan direction extending along the vertical. The X-mirror is oscillated, typically at resonance, at a scan frequency and at a speed that varies along each scan line, and the Y-mirror is driven at a constant speed during a forward scan from an upper to a lower portion of the raster pattern (or vice versa) and is either driven or allowed to self-return during a return scan from the lower to the upper portion of the raster pattern (or vice versa).
The repetitive sweeping of the light beam is performed by a pair of drives, one for each scan mirror. The drives may be the same or different. Typically, one of the drives includes a permanent magnet mounted on a scan mirror for joint oscillation. A feedback coil is positioned adjacent the magnet. In response to a periodic drive signal applied by the drive, the magnet and the mirror are oscillated. A feedback signal is generated by the feedback coil during oscillation. The frequency of the feedback signal is the same as the mirror motion, with one cycle of the feedback signal corresponding to one cycle of mirror motion. The amplitude of the feedback signal is proportional to the velocity of the mirror motion. The polarity of the feedback signal is dependent on the direction of mirror motion such that a positive half cycle of the feedback signal indicates that the mirror is moving in one drive direction, and a negative half cycle indicates that the mirror is moving in the opposite drive direction. Zero crossings of the feedback signal occur when the mirror reaches its maximum travel at each end of a respective scan line. At each zero crossing, the mirror stops for an instant and reverses drive direction.
The feedback signal is useful for various purposes. For example, an electrical drive monitoring circuit is often employed to monitor the amplitude of the feedback signal and, for example, turn the laser beam off if the amplitude falls below a predetermined threshold, thereby indicating that the drive is malfunctioning. An electrical closed loop control circuit is also often employed to process the feedback signal to make decisions about whether to continue energizing the drive. Still another electronic circuit that is often employed processes the zero crossings of the feedback signal to derive a start-of-scan (SOS) signal that represents mirror motion and is used to synchronize the scan lines.
Electro-optical readers are also well known in the art for electro-optically transforming a spatial pattern of graphic indicia, known as a symbol, into a time-varying electrical signal which is then decoded into data. Typically, a light beam generated from a light source is focused by a lens along an optical path toward a target that includes the symbol. The light beam is repetitively swept along a scan line or a series of scan lines arranged in a raster pattern over the symbol by moving one or more scan mirrors located in the optical path. A photodetector detects light scattered or reflected from the symbol and generates an analog electrical signal. Electronic circuitry converts the analog signal into a digitized signal having pulse widths corresponding to physical widths of bars and spaces comprising the symbol, and a decoder decodes the digitized signal into data descriptive of the symbol.
The repetitive sweeping of the light beam in readers is performed by a drive, typically a motor having a rotor oscillatable about an axis. A permanent magnet and the scan mirror are jointly oscillatable with the rotor. The motor is driven by a drive coil wound on a bobbin that is located physically close to the permanent magnet. A feedback coil is also wound on the same bobbin. In response to an alternating voltage drive signal applied to the drive coil, the electromagnetic field produced by the drive coil interacts with the permanent magnetic field of the magnet, thereby jointly moving the magnet and the mirror.
Although generally satisfactory for its intended purpose, the feedback signal in image projectors and electro-optical readers can be contaminated by the periodic drive signal voltage, as well as by the switching electronics for producing the periodic drive signal voltage. The periodic drive signal voltage couples to the feedback coil and adds to the voltage of the feedback signal. Since the contaminants are synchronous with the feedback signal, it is not readily possible to remove the contaminants by signal processing. Hence, the position of the mirror cannot be precisely located. Such contamination is a problem in electro-optical readers and is a severe problem in image projectors, because the motion or velocity of the scan mirror and, hence, of each scan line swept by the scan mirror must be very highly controlled to be a constant value for both right-to-left and left-to-right scan lines. Otherwise, the projected image will be degraded.